JP4412188B2 - Concrete pillar and its construction method - Google Patents

Description

The present invention relates to a concrete column and a construction method thereof, and more particularly to a concrete column using high-strength concrete having a design standard strength exceeding 100 [N / mm ² ] and a construction method thereof.

In recent years, high-strength concrete has been widely used with the rise of buildings and long-term use. High-strength concrete can be expected to be used for a long period of time as a structure on the lower floors of super-high-rise buildings and high-rise buildings, and is therefore used for structures with high durability. In particular, the so-called ultra-high-strength concrete, in which the height of the building has been increased and the design standard strength exceeds 100 [N / mm ² ] has been used for the actual building structure in recent years.
However, since high-strength concrete has a low moisture content, it tends to cause an explosion phenomenon that explodes when heated violently by a fire or the like. Therefore, in order to prevent the explosion phenomenon, a method for constructing a reinforced concrete column in which a cover concrete having a design strength lower than that of the high-strength concrete is arranged around the core concrete made of ultra-high-strength concrete is disclosed. (For example, see Patent Document 1)
JP 2004-232233 A

However, high-strength concrete having a design standard strength exceeding 100 [N / mm ² ] causes brittle fracture when subjected to deformation exceeding the strain at the maximum compressive strength. For this reason, if deformation exceeding the degree of distortion at the maximum compressive strength is applied to a reinforced concrete column using high-strength concrete with a design standard strength exceeding 100 [N / mm ² ], the cover concrete outside the shear reinforcement will be It causes brittle fracture and peels off. As a result, before reaching the ultimate bending strength of the column cross section determined in consideration of the concrete design standard strength and the yield of the reinforcing bars, the cover concrete can no longer bear the compressive force, and the strength of the concrete column will decrease. .

Accordingly, an object of the present invention is to provide a concrete column in which the covering concrete does not cause brittle fracture or peeling and does not cause a decrease in strength while using high-strength concrete having a design standard strength of 100 [N / mm ² ] or more. That is.

Concrete pole of the present invention, the periphery of the core concrete design strength consists 100 [N / mm ^2] or more high strength concrete, design strength is 40 [N / mm ^2] or more, and, 100 [N / together mm ^2] or less, wherein the difference between the design strength of the core concrete provided 80 [N / mm ^2] the concrete cover consisting of at which the concrete to the core concrete and integral. Here, the cover concrete is preferably a precast concrete formwork.

According to the above invention, the concrete of 100 [N / mm ² ] or less does not cause brittle fracture even when subjected to deformation exceeding the strain at the maximum compressive strength, and the covering concrete does not peel off, so that the strength does not decrease. A stable load-strain relationship can be obtained.

In addition, the present invention is a method for constructing a concrete column as described above, and has a design standard strength of 40 [N / mm ² ] or more and 100 [N / mm ² ] or less , and the design standard of the core concrete. A precast concrete formwork made of concrete having a difference from the strength of 80 [N / mm ² ] or less is installed, and the design standard strength is 100 [N / mm ² ] or more in the precast concrete formwork, and 200 [N / mm ² ] The following high-strength concrete is placed, and the precast concrete formwork is left, and a concrete column construction method is also included.
The concrete column of the present invention has a maximum compressive stress strain of 3000 [μ] or less around a core concrete made of high strength concrete having a maximum compressive stress strain of 3000 [μ] or more. Cover concrete made of concrete having a standard strength of 40 [N / mm ² ] or more and a difference from the design standard strength of the core concrete of 80 [N / mm ² ] or less is provided integrally with the core concrete. It is characterized by.
In addition, the concrete pillar in this invention shall mean either a reinforced concrete pillar, a reinforced steel concrete pillar, or a steel concrete pillar.

  In concrete columns, brittle fracture of the cover concrete outside the shear reinforcement can be prevented, so that a stable load-strain relationship can be obtained without a decrease in strength.

First, the properties of the concrete used in the present invention will be described. As an approximate expression representing the relationship between the compressive stress degree σ-strain degree ε of concrete, the Popovics equation shown in the equation (1) is widely used.
Here, S = σ / Fc, X = ε / ε ₀ , n is a constant obtained by experiment, σ is the compressive stress, Fc is the design reference strength, ε is the compressive strain, and ε ₀ is the maximum compressive strain. Show.

Fig. 1 also shows the maximum compressive stress _C σ _B and maximum compressive stress proposed by the Ministry of Construction Comprehensive Technology Development Project, Development of Ultralight and High-rise Technology for Reinforced Concrete Buildings (hereinafter referred to as NewRC). The graph showing the relationship with the compressive strain _C ε _B (referred to as the maximum compressive stress strain) is shown. Thus, if equal design strength Fc and the maximum compressive strength _C sigma _B, the relationship between the maximum compressive strength _C sigma _B and the maximum compressive stress at skewness _C epsilon _B wherein the NewRC proposes, desired design If the maximum compressive stress strain _C ε _B at the reference strength Fc is obtained, and the maximum compressive stress strain _C ε _B is equal to the maximum compressive strain ε _{0, the} stress at each design reference strength Fc is obtained from the equation (1). A relationship of degree σ-strain degree ε is obtained.

Here, an experiment conducted for examining whether the relationship of the stress degree σ-strain degree ε obtained by the above calculation is equal to the actual stress degree σ-strain degree ε relationship of concrete will be described. In this experiment, concrete members having design standard strengths Fc of 55 [N / mm ² ], 80 [N / mm ² ], 90 [N / mm ² ], and 140 [N / mm ² ] were examined by a compressive strength test. The relationship of the stress degree σ-strain degree ε was compared with the relationship of the stress degree σ-strain degree ε obtained by the above calculation. 2 to 5 show the experiment and the above in concrete members having design standard strengths of 55 [N / mm ² ], 80 [N / mm ² ], 90 [N / mm ² ], and 140 [N / mm ² ], respectively. It is a graph which shows the stress degree (sigma) -strain degree (epsilon) curve obtained by calculation.

As shown in FIG. 2, in the concrete member having the design standard strength Fc of 55 [N / mm ² ], the stress degree σ of the experimental result is obtained by the above calculation method at all the compressive strain degrees ε. The value is almost equal to the stress level σ. As a result, it can be understood that the stress degree σ obtained by the above calculation method can be borne within the range of the compressive strain ε in which the experiment was performed.

In the case of a concrete member having a design standard strength Fc of 80 [N / mm ² ] or 90 [N / mm ² ], as shown in FIGS. 3 and 4, the compressive strain ε is the maximum compressive stress strain. _{If C} ε _B or less, the stress level σ of the experimental result and the stress level σ obtained by the calculation are almost equal. When the compressive strain ε exceeds the maximum compressive stress strain _C ε _B , the stress σ of the experimental result is larger than the stress σ obtained by calculation. As a result, it can be seen that the stress degree σ obtained by calculation or a stress degree σ greater than that can be borne by all the compressive strain degrees ε in which the experiment was performed.

However, in the case of a concrete member using high-strength concrete having a design standard strength Fc of 140 [N / mm ² ], the compressive strain ε is not more than the maximum compressive stress strain _C ε _B as shown in FIG. In the range, although the stress value σ of the experimental result and the stress value σ obtained by the calculation are substantially equal, when the compressive strain ε exceeds the maximum compressive stress strain _C ε _B , it becomes 0 [N / mm ² ]. End up. This is because the concrete member causes brittle fracture, and it can be seen that when the compressive strain ε is equal to or greater than the maximum compressive stress strain _C ε _B , the compressive stress σ cannot be borne at all. The maximum compressive stress strain _C ε _B at this time is about 3000 [μ].

The above experiments revealed the following properties of concrete. A concrete member having a design standard strength Fc of 100 [N / mm ² ] or less bears a stress degree σ that is almost equal to the value obtained by calculation when the compressive strain ε is less than the maximum compressive stress strain _C ε _B. it can. Even if the compressive strain ε exceeds the maximum compressive stress strain _C ε _B , the stress σ obtained by calculation or a stress σ greater than that can be borne. However, a concrete member having a design standard strength Fc of 100 [N / mm ² ] or more causes brittle fracture when the compressive strain ε exceeds the maximum compressive stress strain _C ε _B (= approximately 3000 [μ]). Therefore, the compressive stress degree σ cannot be borne at all.

Therefore, in the present invention, the concrete column 1 using high-strength concrete is configured as follows. FIG. 6 shows a partially broken perspective view of the concrete column 1 of the present invention, and FIG. 7 shows a horizontal sectional view of the reinforced concrete column 1. This reinforced concrete column 1 is composed of an outer shell precast concrete pipe (hereinafter referred to as outer shell precast) 20 which is a hollow rectangular tube-shaped precast concrete mold, and a core concrete 10 placed in a hollow portion of the outer shell precast 20. The The outer shell precast 20 includes a concrete 21 having a design standard strength of 40 [N / mm ² ] or more and 100 [N / mm ² ] or less, and a plurality of shear reinforcement bars 22 embedded in the concrete 21. Composed. The core concrete 10 includes a high strength concrete 11 having a design standard strength of 100 [N / mm ² ] or more and 200 [N / mm ² ] or less, and a main reinforcement 12 embedded in the high strength concrete 11. Composed.

The reinforced concrete column 1 is constructed with a precast 20 made of concrete 21 having a design standard strength of 40 [N / mm ² ] or more and 100 [N / mm ² ] or less, and a design standard strength of 100 [N / mm ² ]. N / mm ² ] or more and 200 [N / mm ² ] or less high-strength concrete 11 is placed, and the outer shell precast 20 is left as it is.

Next, a mechanism for improving the strength of the reinforced concrete column 1 by the above structure will be described. FIG. 8 is a diagram showing a situation in which a bending moment is acting on the cross section of the reinforced concrete column 1 in which high-strength concrete is cast on the entire cross section. It shows the case is less than _{C ε B (= 3000 [μ} ]). 8A is a horizontal sectional view of a reinforced concrete column, FIG. 8B is a diagram showing an axial strain distribution of this cross section, and FIG. 8C is a stress degree acting on the cross section. It is a figure which shows distribution. When bending stress is applied to the cross section of the reinforced concrete column 1, if the assumption that the cross section that was a plane before the deformation is still a plane after the deformation is used, as shown in FIG. It can be assumed that the skewness has a linear distribution. Therefore, the strain ε is maximized in the cross section at the outermost edge on the side receiving the stress σ (this strain ε is defined as the maximum compressive strain ε _max ). Further, the stress degree distribution can be obtained from the strain degree distribution in the cross section by using the above-described calculation of the relationship of the stress degree σ-strain degree ε of the concrete. As shown in FIG. 8C, it can be seen that stress can be borne in the entire cross section when the stress distribution is seen.

Next, the bending stress acting on the cross section increases, the cross section curvature increases, and the degree of distortion of the cover concrete outside the shear reinforcement exceeds the maximum compressive stress strain _C ε _B (= 3000 [μ]). explain. FIG. 9A is a horizontal sectional view of the reinforced concrete column 1 when the degree of distortion of the cover concrete outside the shear reinforcement exceeds the strain _C ε _B (= 3000 [μ]) at the maximum compressive stress, (b ) Is a diagram showing the skewness distribution of the cross section at this time, and (c) is a diagram showing the stress degree distribution acting on the cross section at this time. If the degree of strain ε exceeds the maximum strain _C ε _B (= 3000 [μ]) at the time of compressive stress, the cover concrete peels due to brittle fracture and cannot bear the compressive stress. Therefore, as shown in FIG. 9 (c), the compressive stress degree of the cover concrete having the strain ε exceeding 3000 [μ] is 0 [N / mm ² ], and only the core concrete bears the stress degree σ. For this reason, the strength of the reinforced concrete column is reduced.

Next, the case where the outer shell precast of the reinforced concrete column 1 is subjected to a bending stress that causes a strain exceeding the maximum compressive stress strain _C ε _B (= 3000 [μ]) will be described. (A) of FIG. 10 is a horizontal sectional view of the reinforced concrete column 1 when a bending stress causing a strain exceeding the maximum compressive stress strain _C ε _B (= 3000 [μ]) is applied, ) Is a diagram showing the skewness distribution of the cross section at this time, and (c) is a diagram showing the stress degree distribution acting on the cross section. As shown in (b), even if a portion of the outer shell precast that produces a strain ε exceeding the maximum compressive stress strain _C ε _B (= 3000 [μ]) appears, the outer shell precast has a design standard strength of 100. [N / mm ² ] Brittle fracture does not occur because it is made of the following concrete. For this reason, as shown in FIG. 10C, even when a strain exceeding the maximum compressive stress strain _C ε _B (= 3000 [μ]) is generated, the outer shell precast can bear the stress. The strength is improved compared to a reinforced concrete column in which concrete with a design standard strength of 100 [N / mm ² ] or more is placed on the cross section.

  However, when the design standard strength of the concrete used for the outer shell precast is extremely lower than the design standard strength of the core concrete, the strength is higher than when concrete of the design standard strength equal to the core concrete is placed on the entire cross section. May fall. This is because the use of concrete with extremely low strength for the outer shell precast has the effect of lowering strength by using extremely low strength concrete than the effect of improving strength by preventing brittle fracture. This is because the strength becomes large when viewed as a whole reinforced concrete column.

Therefore, the design standard strength of the concrete used for the outer shell precast was examined by cross-sectional analysis. Table 1 is a table showing various conditions of reinforced concrete columns subjected to cross-sectional analysis.

As shown in Table 1, when the design standard strength of all sections is 120 [N / mm ² ] (comparative example), the design standard strength of core concrete is 120 [N / mm ² ], and the design standard of outer shell precast When the strength is 80 [N / mm ² ] (Example 1), the design standard strength of the core concrete is 120 [N / mm ² ], and the design standard strength of the outer shell precast is 60 [N / mm ² ]. In the case (Example 2), when the design standard strength of the core concrete is 120 [N / mm ² ] and the design standard strength of the outer shell precast is 40 [N / mm ² ] (Example 3), the cross-sectional analysis is performed. The relationship between the curvature of the cross section and the moment under each condition and the maximum moment were obtained. The cross section of the reinforced concrete column was 360 [mm] x 360 [mm] in depth, the core concrete was 290 [mm] x 290 [mm] in depth, the rebar was D19 (SD685), and the axial force was 370 [tonf]. .

The obtained results are shown in FIG. FIG. 11 is a graph showing the relationship between the curvature and moment for the comparative example and Examples 1 to 3, and Table 2 is a table showing the maximum moment of each condition.

As shown in FIG. 11, when high-strength concrete having a design standard strength of 120 [N / mm ² ] is placed on the entire cross section (comparative example), the moment to be increased increases as the curvature increases. However, when the curvature exceeds about 0.00018, brittle fracture occurs from the outermost edge on the compression side. Further, when the curvature increases, brittle fracture proceeds in the direction of the neutral axis of the cross section, the cover concrete is peeled off, and the compressive stress cannot be borne, so the borne moment is reduced. And, the curvature is about 0.00021, and the cover concrete is all brittlely broken. If the curvature exceeds about 0.00021, the inside of the shear reinforcement is constrained and has toughness, so that the moment gradually increases without causing brittle fracture. Thus, when high-strength concrete with a design standard strength of 120 [N / mm ² ] is placed on the entire cross section, the load-deformation relationship is very unstable. The moment is maximized immediately before brittle fracture occurs, and the moment at this time is about 63 [tonf-m].

On the other hand, concrete with a design standard strength of 120 [N / mm ² ] is used for the core concrete, and design standard strengths of 40 [N / mm ² ], 60 [N / mm ² ] for the outer shell precast, When 80 [N / mm ² ] concrete is used (Examples 1 to 3), the curvature is increased, and brittle fracture does not occur even when the outermost edge becomes large. Therefore, the moment increases as the curvature increases. It will increase. Thus, in Examples 1 to 3, since brittle fracture does not occur and there is no sudden load drop, a stable load-deformation relationship can be obtained. Further, when the maximum moment is compared, it is about 68 [tonf-m] in Example 1, which is larger than that in the comparative example. Moreover, Example 2 is about 63 [tonf-m], which is the same value as the comparative example. However, Example 3 is about 58 [tonf-m], which is lower than the comparative example.

Thus, the difference between the design standard strength of the core concrete of Examples 1 to 3 used in this study and the design standard strength of the outer shell precast is 40 [N / mm ² ], 60 [N / mm ² ], respectively. 80 [N / mm ² ], and the improvement in strength is seen in Example 1, but the strength in Example 2 is the same as that in the comparative example, and in the case of Example 3, the strength is reduced. It has become.

For these reasons, although it depends on the difference in various conditions, if the difference between the design standard strength of the core concrete and the design standard strength of the outer shell precast is 60 [N / mm ² ] or less, the same design standard strength is used for all sections. It can be seen that the strength is improved as compared with the case of placing concrete. Therefore, high strength concrete with a design standard strength of 100 [N / mm ² ] or more is used for the core concrete, so that the concrete to be cast on the outer shell precast has a design standard strength of 40 [N / mm ² ] or more. Need to be. Focusing on the load-deformation relationship, when high-strength concrete with a design standard strength of 100 [N / mm ² ] or more is used for the entire cross-section, the degree of distortion at the outermost edge of the concrete member is the degree of strain at the maximum compressive stress. _Although it was a very unstable load-deformation relationship in which the strength suddenly decreased when _C ε _B (= 3000 [μ]) was exceeded, the concrete column of the present invention has a rapid strength even when the strain increases. It was found that a stable load-deformation relationship could be obtained.

  Next, the reinforced concrete column 1 having the above configuration will be described because it has been confirmed by a column bending shear experiment that the strength is increased. FIG. 12A is a front sectional view of the test specimen, and FIG. 12B is a transverse sectional view of the specimen. Table 3 is a table showing various conditions of the specimen. As shown in FIG. 12, the test body 30 includes a reinforced concrete column 1 and a stub 32 joined to the upper and lower ends of the reinforced concrete column 1, and the reinforced concrete column 1 is fixedly supported by the stub 32. While applying the axial force from the top and bottom of the test body using a hydraulic jack and applying the horizontal force to the upper stub with another hydraulic jack while the lower stub is fixed, the reinforced concrete column 1 of the test body is applied. Shear force and anti-symmetric moment were generated, and the interlayer deformation angle was increased stepwise alternately.

As an example of the present invention, the outer shell precast is made of concrete having a design standard strength of 60 [N / mm ² ], and a high strength concrete having a design standard strength of 120 [N / mm ² ] is placed therein. As a comparative example, a test body (comparative example) in which high-strength concrete having a design standard strength of 120 [N / mm ² ] was placed on the entire cross section of the column was used. The other conditions were the same as shown in Table 3.

In addition, Table 4 is a table | surface which shows the actual compressive strength of the concrete used for each part of the test body used for experiment, and was used for the high-strength concrete used for the column cross section of the comparative example, and the core concrete of an Example. It was confirmed that high-strength concrete has almost the same compressive strength.

  FIG. 13 is a graph showing the shear force and deformation history of the comparative example, and FIG. 14 is a graph showing the shear force and deformation history curve of the example. As shown in FIG. 13, in the reinforced concrete column (comparative example) in which high-strength concrete is placed on the entire cross section, the shear force increases as the interlayer displacement increases until the interlayer displacement reaches 6.0 [mm]. Go. However, when the interlayer displacement exceeded 6.0 [mm], brittle fracture occurred from the outermost edge, and the shear force hardly changed even when the interlayer displacement was increased. The shearing force at this time was a value almost equal to the calculated yield strength when the outermost edge distortion was 0.3% (= 3000 [μ]).

On the other hand, in the example, since the concrete with a design standard strength of 100 [N / mm ² ] or less is used for the outer shell precast, there is no brittle fracture. For this reason, in the embodiment, as shown in FIG. 14, even when the interlayer displacement exceeds 6.0 [mm], the load-deformation relationship does not become unstable, and the shear force increases with the increase of the interlayer displacement. Increasing stable load-deformation relationship was obtained. When the maximum shear force is compared, the maximum shear strength of the comparative example is 1313 [kN], but the maximum shear strength of the example is 1429 [kN], which indicates that the shear strength is improved.

  FIG. 15 is a diagram showing a damage situation of the reinforced concrete columns of the comparative example and the example when the interlayer displacement is 5.4 [mm], 10.8 [mm], and 21.6 [mm]. The solid line shows the cracks that occurred in the reinforced concrete columns, and the colored parts show the parts that have undergone brittle fracture and have peeled off. As shown in FIG. 15, in all cases where the interlayer displacement is 5.4 [mm], 10.8 [mm], and 21.6 [mm], the example has fewer cracks and peeling than the comparative example. From this, it can be seen that the strength of the reinforced concrete column is improved because the embodiment can suppress brittle fracture.

Based on the above experiment, around the core concrete made of concrete having a design standard strength of 100 [N / mm ² ] or more, from concrete having a design standard strength of 40 [N / mm ² ] or more and 100 [N / mm ² ] or less. Reinforced concrete columns with outer shell precasts can reduce the brittle fracture of cover concrete, so that the shear strength and strength of the reinforced concrete columns with the design standard strength of 100 [N / mm ² ] or more in all sections are higher. It was confirmed that the bending strength could be improved and a stable load-deformation relationship could be obtained.

  In addition, although the said embodiment demonstrated the case where this invention was applied to the reinforced concrete pillar, it can apply not only to this but to a reinforced steel concrete pillar or a steel concrete pillar.

It is a graph showing the relationship between the maximum compressive stress degree σ ₀ proposed by New RC and the compressive strain degree ε ₁ at the maximum compressive stress degree. It is a graph which shows the stress degree (sigma) -strain degree (epsilon) curve calculated | required by experiment and calculation of the concrete member whose design reference | standard intensity | strength Fc is 55 [N / mm < ² >]. It is a graph which shows the stress degree (sigma) -strain degree (epsilon) curve calculated | required by experiment and calculation of the concrete member whose design reference | standard intensity | strength Fc is 80 [N / mm < ² >]. It is a graph which shows the stress degree (sigma) -strain degree (epsilon) curve calculated | required by experiment and calculation of the concrete member whose design reference | standard intensity | strength Fc is 90 [N / mm < ² >]. It is a graph which shows the stress degree (sigma) -strain degree (epsilon) curve calculated | required by experiment and calculation of the concrete member whose design reference | standard intensity | strength Fc is 140 [N / mm < ² >]. It is a partially broken perspective view of the concrete pillar of this invention. It is a horizontal sectional view of the concrete pillar of the present invention. (A) is a horizontal cross-sectional view of a column in which high-strength concrete is cast on the entire cross section, (b) is a diagram showing the distribution of strain ε of this cross section, and (c) is the stress acting on the cross section. It is a figure which shows distribution of (sigma). (A) is a horizontal cross-sectional view of a column in which high-strength concrete is cast on the entire cross section, (b) is a diagram showing the distribution of strain ε of this cross section, and (c) is the stress acting on the cross section. It is a figure which shows distribution of (sigma). (A) is a horizontal direction sectional view of the reinforced concrete column of the present invention, (b) is a diagram showing the distribution of strain ε of this section, (c) is a diagram showing the distribution of stress σ acting on the section It is. It is a graph which shows the relationship between the curvature and moment about the comparative example and Examples 1-3 which were calculated | required by cross-sectional analysis. (A) is front sectional drawing of the test body of the column bending shear experiment, (b) is sectional drawing of a side surface direction. It is a graph which shows the relationship between the shear force of a comparative example, and a deformation | transformation. It is a graph which shows the relationship between the shear force of an Example, and a deformation | transformation. It is a figure which shows the damage condition of a comparative example and an Example.

Explanation of symbols

1 Reinforced concrete pillar 10 Core concrete 11 High-strength concrete 12 Main reinforcement 20 Outer shell precast concrete pipe (precast concrete formwork)
21 Concrete 22 Shear reinforcement

Claims (4)

Around the core concrete design strength consists 100 [N / mm ^2] or more high-strength concrete,
A concrete having a design standard strength of 40 [N / mm ² ] or more and 100 [N / mm ² ] or less and a difference from the design standard strength of the core concrete of 80 [N / mm ² ] or less. A concrete column comprising a cover concrete formed integrally with the core concrete.   The concrete column according to claim 1, wherein the cover concrete is a precast concrete formwork. 3. The method for constructing a concrete column according to claim 2, wherein the design standard strength is 40 [N / mm ² ] or more and 100 [N / mm ² ] or less , and the design standard strength of the core concrete is A precast concrete formwork made of concrete having a difference of 80 [N / mm ² ] or less is installed, and the design standard strength is 100 [N / mm ² ] or more and 200 [N / mm] in the precast concrete formwork. mm ² ] A method for constructing a concrete column, wherein the following high-strength concrete is cast and the precast concrete formwork is left behind. Around the core concrete made of high-strength concrete with maximum compressive stress strain of 3000 [μ] or more,
Maximum compressive stress distortion is 3000 [μ] or less and design standard strength is 40 [N / mm ² ] And the difference from the design standard strength of the core concrete is 80 [N / mm] ² ] A concrete column characterized in that a cover concrete made of the following concrete is provided integrally with the core concrete.